Dockerized Python Web Application Can't Connect to PostgreSQL Database: Connection Refused Error

Question

I am trying to set up a Docker environment for my Python web application. I have configured Nginx as a reverse proxy, Gunicorn as a WSGI HTTP server, and I’m using Redis for caching and PostgreSQL as my database. I’ve also tried to implement load balancing with Nginx and I’m using Docker Compose for orchestrating all these services.

Everything seemed to be configured correctly, but when I try to run the application, I’m experiencing an issue where my application isn’t communicating with the PostgreSQL database. The application logs are indicating a connection error. Here’s the error I’m seeing:

psycopg2.OperationalError: could not connect to server: Connection refused Is the server running on host "db" (172.20.0.2) and accepting TCP/IP connections on port 5432?

I’ve checked my Docker Compose file and everything seems to be in order. Here’s a part of my docker-compose.yml:

version: '3'
services:
  db:
    image: postgres
    volumes:
      - ./data/db:/var/lib/postgresql/data
    environment:
      POSTGRES_DB: mydb
      POSTGRES_USER: myuser
      POSTGRES_PASSWORD: mypassword

  web:
    build: .
    command: gunicorn myapp:app -c ./gunicorn_config.py
    volumes:
      - .:/code
    ports:
      - "8000:8000"
    depends_on:
      - db

And this is how I’m trying to connect to the database in my Python application:

import psycopg2
conn = psycopg2.connect(
    dbname="mydb",
    user="myuser",
    password="mypassword",
    host="db"
)

I can’t figure out what I’m doing wrong. Any ideas why the application is unable to connect to the PostgreSQL database?

KFSys · Accepted Answer

Hello there,

There could be a couple of issues here, but I’ll start with the most common one when dealing with Docker and databases and you can let me know how it goes.

The problem is likely that your web application is starting before your PostgreSQL database is fully ready to accept connections. Even though you’ve specified depends_on in your docker-compose.yml, this only ensures that the db service starts before the web service. It doesn’t wait for the db service to be “ready” before starting web.

To solve this issue, you can use a simple script that checks the database connection before starting your application. Here’s a simple Python script that does exactly that:

import time
import psycopg2
from psycopg2 import OperationalError

def create_conn():
    conn = None
    while not conn:
        try:
            conn = psycopg2.connect(
                dbname="mydb",
                user="myuser",
                password="mypassword",
                host="db"
            )
            print("Database connection successful")
        except OperationalError as e:
            print(e)
            time.sleep(5)
    return conn

conn = create_conn()

In this script, we’re using a while loop to try to establish a connection to the database. If it can’t connect, it will wait for 5 seconds before trying again. Once the connection is established, it will break the loop and proceed.

You can adjust this script to your needs, for instance, by limiting the number of connection attempts or increasing the sleep time.

Another approach would be to use a tool like wait-for-it or dockerize which can pause the execution of your application until the db service is ready to accept connections.

If this doesn’t solve your problem, let me know. It might be a more specific issue with your configuration or environment.

Hope that helps!

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Dockerized Python Web Application Can't Connect to PostgreSQL Database: Connection Refused Error

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